Minecraft has, to date, sold over 600,000 copies and is seen by some as the next great MMO-gaming experience. There are currently over 1000 servers for the game, nearly 2 million registered users (the game is also available for free), and has seen one of the fastest growth spurts of any game in history. At its' core, Minecraft is a sandbox construction game, where players can build anything they can imagine (See Curse logo above). The more you learn about the game, the more you see it's much more than that, it's a place where players are free to build buildings, bases, bridges, and battle an endless horde of creatures.
Here's a video of a giant being slain, and a sample of how farming works in Minecraft: